An official website of the United States government
Abstract Determining the injection of glacial meltwater into polar oceans is crucial for quantifying the climate system response to ice sheet mass loss. However, meltwater is poorly observed and its pathways poorly known, especially in winter. Here we present winter meltwater distribution near Pine Island Glacier using data collected by tagged seals, revealing a highly variable meltwater distribution with two meltwater-rich layers in the upper 250 m and at around 450 m, connected by scattered meltwater-rich columns. We show that the hydrographic signature of meltwater is clearest in winter, when its presence can be unambiguously mapped. We argue that the buoyant meltwater provides near-surface heat that helps to maintain polynyas close to ice shelves. The meltwater feedback onto polynyas and air-sea heat fluxes demonstrates that although the processes determining the distribution of meltwater are small-scale, they are important to represent in Earth system models.
more »
« less
On the settling depth of meltwater escaping from beneath Antarctic ice shelves
Abstract Antarctic glacial meltwater is thought to play an important role in determining large-scale Southern Ocean climate trends, yet recent modeling efforts have proceeded without a good understanding of how its vertical distribution in the water column is set. To rectify this, here we conduct new large-eddy simulations of the ascent of a buoyant meltwater plume after its escape from beneath an Antarctic ice shelf. We find that the meltwater’s settling depth is primarily a function of the buoyancy forcing per unit width of the source and the ambient stratification, consistent with the classical theory of turbulent buoyant plumes and in contrast to previous work that suggested an important role for centrifugal instability. Our results further highlight the significant role played by localized variability in stratification; this helps explain observed interannual variability in the vertical meltwater distribution near Pine Island Glacier. Because of the vast heterogeneity in mass loss rates and ambient conditions at different Antarctic ice shelves, a dynamic parameterization of meltwater settling depth may be crucial for accurately simulating high-latitude climate in a warming world; we discuss how this may be developed following this work, and where the remaining challenges lie.
more »
« less
- Award ID(s):
- 1643285
- PAR ID:
- 10344014
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- ISSN:
- 0022-3670
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Southern Ocean surface cooling and Antarctic sea ice expansion from 1979 through 2015 have been linked both to changing atmospheric circulation and melting of Antarctica's grounded ice and ice shelves. However, climate models have largely been unable to reproduce this behavior. Here we examine the contribution of observed wind variability and Antarctic meltwater to Southern Ocean sea surface temperature (SST) and Antarctic sea ice. The free‐running, CMIP6‐class GISS‐E2.1‐G climate model can simulate regional cooling and neutral sea ice trends due to internal variability, but they are unlikely. Constraining the model to observed winds and meltwater fluxes from 1990 through 2021 gives SST variability and trends consistent with observations. Meltwater and winds contribute a similar amount to the SST trend, and winds contribute more to the sea ice trend than meltwater. However, while the constrained model captures much of the observed sea ice variability, it only partially captures the post‐2015 sea ice reduction.more » « less
-
Abstract. Over recent decades Antarctic sea-ice extent has increased, alongsidewidespread ice shelf thinning and freshening of waters along the Antarcticmargin. In contrast, Earth system models generally simulate a decrease insea ice. Circulation of water masses beneath large-cavity ice shelves is notincluded in current Earth System models and may be a driver of thisphenomena. We examine a Holocene sediment core off East Antarctica thatrecords the Neoglacial transition, the last major baseline shift ofAntarctic sea ice, and part of a late-Holocene global cooling trend. Weprovide a multi-proxy record of Holocene glacial meltwater input, sedimenttransport, and sea-ice variability. Our record, supported by high-resolutionocean modelling, shows that a rapid Antarctic sea-ice increase during themid-Holocene (∼ 4.5 ka) occurred against a backdrop ofincreasing glacial meltwater input and gradual climate warming. We suggestthat mid-Holocene ice shelf cavity expansion led to cooling of surfacewaters and sea-ice growth that slowed basal ice shelf melting.Incorporating this feedback mechanism into global climate models will beimportant for future projections of Antarctic changes.more » « less
-
Abstract. Antarctic meltwater is a significant source of iron that fertilizes present-day Southern Ocean ecosystems and may enhance marine carbon burial on geologic timescales. However, it remains uncertain how the nutrient flux from the subglacial system changes through time, particularly in response to climate, due to an absence of geologic records detailing element mobilization beneath ice sheets. In this study, we present a 25 kyr record of aqueous trace metal cycling in subglacial water beneath the David Glacier catchment measured in a subglacial chemical precipitate that formed across glacial termination III (TIII), from 259.5 to 225 ka. The deposition rate and texture of this sample describe a shift in subglacial meltwater flow following the termination. Alternating layers of opal and calcite deposited in the 10 kyr prior to TIII record centennial-scale subglacial flushing events, whereas reduced basal flushing resulted in slower deposition of a trace-metal-rich (Fe, Mn, Mo, Cu) calcite in the 15 kyr after TIII. This sharp increase in calcite metal concentrations following TIII indicates that restricted influx of oxygen from basal ice melt to precipitate-forming waters caused dissolution of redox-sensitive elements from the bedrock substrate. The link between metal concentrations and climate change in this single location across TIII suggests that ice motion may play an important role in subglacial metal mobilization and discharge, whereby heightened basal meltwater flow during terminations supplies oxygen to subglacial waters along the ice sheet periphery, reducing the solubility of redox-sensitive elements. As the climate cools, thinner ice and slower ice flow decrease subglacial meltwater production rates, limiting oxygen delivery and promoting more efficient mobilization of subglacial trace metals. Using a simple model to calculate the concentration of Fe in Antarctic basal water through time, we show that the rate of Antarctic iron discharge to the Southern Ocean is sensitive to this heightened mobility and may therefore increase significantly during cold climate periods.more » « less
-
Leads play an important role in the exchange of heat, gases, vapour, and particles between seawater and the atmosphere in ice-covered polar oceans. In summer, these processes can be modified significantly by the formation of a meltwater layer at the surface, yet we know little about the dynamics of meltwater layer formation and persistence. During the drift campaign of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), we examined how variation in lead width, re-freezing, and mixing events affected the vertical structure of lead waters during late summer in the central Arctic. At the beginning of the 4-week survey period, a meltwater layer occupied the surface 0.8 m of the lead, and temperature and salinity showed strong vertical gradients. Stable oxygen isotopes indicate that the meltwater consisted mainly of sea ice meltwater rather than snow meltwater. During the first half of the survey period (before freezing), the meltwater layer thickness decreased rapidly as lead width increased and stretched the layer horizontally. During the latter half of the survey period (after freezing of the lead surface), stratification weakened and the meltwater layer became thinner before disappearing completely due to surface ice formation and mixing processes. Removal of meltwater during surface ice formation explained about 43% of the reduction in thickness of the meltwater layer. The remaining approximate 57% could be explained by mixing within the water column initiated by disturbance of the lower boundary of the meltwater layer through wind-induced ice floe drift. These results indicate that rapid, dynamic changes to lead water structure can have potentially significant effects on the exchange of physical and biogeochemical components throughout the atmosphere–lead–underlying seawater system.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JPO-D-20-0286.1
